The present invention relates to methods and apparatus for producing a precise and accurate current value at a remote location in response to a programmed current at a local location.
Accurate and precise current values are desirable in a number of applications, including digital-to-analog conversion, image display driving, etc.
For example, in an organic light emitting diode (OLED) display, a plurality of pixels are arranged in rows and columns, where each pixel includes two thin film transistors (TFTs), one an addressing (or switching) transistor and the other a driving (or power) transistor, a storage capacitor, and an OLED device. For activation of a given pixel of the OLED array, a scan line (row line) is selected, and a video signal is loaded on a data line (column line) and input to the driving transistor (via the addressing transistor) to control a current through the OLED device. The video signal is stored on the storage capacitor for the duration of one frame.
An OLED device emits light at intensities proportional to the currents that pass through the device. Therefore, current drive is the preferred OLED driving mode. There are, however, at least two problems that have plagued the OLED display driver industry. The wide dynamic range in OLED pixels requires very small currents at the low end of OLED luminance. The distribution of small, precise currents to remote pixel locations in the OLED array may be corrupted by systemic offset errors and leakage currents leading to non-uniform display luminance. In addition, small currents do not provide adequate drive to quickly settle voltages on column lines with significant distributed capacitance. Thus, the ability to establish the pixel illuminations for the entire array within the time available for a given video frame may be impacted. The above problems are exacerbated as display resolutions increase. Indeed, the available settling times for the array pixels reduce as the resolution increases.
Conventional display driver technology employs thin film transistor circuits to program current or program voltage at the given pixel sites. In current programming, a current is sent to the OLED pixel through a current mirror at the site. In voltage programming, a voltage is converted to a pixel drive current through a pixel drive transistor at the pixel site. These techniques demonstrate reasonable stability but suffer from the aforementioned intensity non-uniformities and slow settling times (particularly at low currents). While voltage programming techniques may tend to settle the pixel site more quickly than current programming, such techniques suffer from systemic transistor mismatches and OLED drive current shifts as the OLED ages.
The problems of illumination non-uniformities and poor settling times have rendered the conventional current techniques for driving OLED arrays unsatisfactory. As a result, the commercial display industry has been slow to adopt OLED technology.
Thus, there is a need in the art for methods and apparatus for providing precise currents to the OLED pixel sites that are accurate over a wide dynamic range, exhibit fast settling times, and maintain accuracy as the OLED devices age.